The present invention relates generally to high density packaging and interconnecting of light emitting diodes (LEDs).
To more effectively use solid state LED-based light sources for lighting applications, a low-cost packaging and interconnect technology must be developed. Substrates used in the growth of blue-green LEDs such as GaN (gallium nitride)-based compound semiconductor devices are typically insulating materials such as aluminum oxide (Al.sub.2 O.sub.3). Because a substrate is an insulating material, the electrical contacts are fabricated on the top surface of the device. This type of packaging and interconnect configuration has the drawback of low device packaging density.
In conventional packaging methods, such as Nakamura et al., U.S. Pat. No. 5,563,422, issued 8 Oct. 1996, wherein a GaN-based device is packaged in a housing unit with contact electrodes, the LED device has a transparent and electrically insulating substrate. After contacts are fabricated on the LED, bonding pads are formed of gold, and the device is mounted on top of a lead frame structure. Typically these LED devices are mounted into epoxy domes and situated at a focal point of a parabolic shaped lead frame. Bonding wires can be used to connect LED electrode contacts and the lead frame.
These packages are adequate for applications in which a high packaging density is not required, but several drawbacks result from using lead frames. For example, the device packaging is typically limited to two to three devices per centimeter because of the wire bonds and lead frames. Typically, the lead frame is enclosed inside an epoxy dome having a diameter of a few millimeters. Because the device is placed on top of substrate that is thermally insulated, the only thermal path is the bonding wires in contact with the lead frame. Optical coupling efficiency is reduced because light escaping from the junction has to transmit through the semi-transparent electrode or be bounced from the bottom in contact with the lead frame. Additionally, a two dimensional array of LEDs with a high packaging density is difficult to achieve with wire bonds.